Gas lubricated mechanical face seals, such as those described in U.S. Pat. Nos. 5,496,047 and 8,651,801 (the disclosures of which are incorporated by reference herein) rely on a cushion of seal gas built up between the sealing faces in order to provide lubrication, and thereby inhibit contact between the faces. In many applications, this lubricating gas is the process or product gas that is being compressed or processed by the mechanical energy transmitted by the rotating shaft.
One application of gas lubricated mechanical face seals (also known as “dry seals” or “dry gas seals”) is to provide a seal between a process chamber of a compressor and the outside environment. For example, in the petrochemical industry, centrifugal compressors may be located at intervals along a natural gas pipeline to boost the gas pressure for processing, to counter the effect of flow losses along the transmission pipelines and to generally keep the gas moving towards its destination. These compressors can be used upstream (during exploration and production), midstream (during processing, storage and transportation), or downstream (during natural gas/and petrochemical refining, transmission and distribution) in a petrochemical process. These centrifugal compressors can also be used to transport other fluids.
To move natural gas or other fluids, centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, thereby increasing the velocity of the gas. A diffuser (divergent duct) converts the velocity energy to pressure energy.
Dry gas seals may be used to reduce frictional wear on the rotating components while preventing leakage of the centrifuged or processed gas. To further inhibit leakage of processed gas into the atmosphere, some centrifugal compressors can include a pair of dry gas seals working in tandem.
When used in high-pressure applications, the seal gas (also known as “lubricating gas”) or other can leak in static (or “standby”) conditions where the seal faces are not spinning relative to each other.
FIG. 1 depicts a cross-sectional view of a typical arrangement of a tandem dry seal assembly 10 as known in the art. As is common for seals of this type, seal assembly 10 can seal the shaft 12 of a rotating machine such that seal assembly 10 provides a seal between the process chamber 14 and a shaft bearing cavity 16. The seal assembly 10 can be mounted in a generally cylindrical bore (or “seal chamber”) defined by the process chamber at an axially inboard end and the bearing chamber at an axially outboard end, and extending radially outward to a housing 18.
Seal assembly 10 comprises rotating components operably coupled to shaft 12 and stationary components operably coupled to housing 18.
Annular sleeve 20 comprises an axially extending shaft portion 22 which can be operably coupled to the shaft 12 for rotation therewith. Sleeve 20 can comprise a flange portion 26 extending radially outwardly at the inboard end of the seal assembly 10, for arrangement proximal process chamber 14. As depicted in FIG. 1, shaft portion 22 extends axially outboard of flange portion 26, however as depicted in FIG. 11, shaft portion 22 can extend axially inboard of flange portion 26 in various seal assemblies.
An annular rotating seal ring (or “mating ring”) 30 is mounted adjacent the outboard face of flange formation 26. An axially fixed annular member 34 can be mounted on the external diameter of shaft portion 22, abutting rotating seal ring 30, thereby inhibiting axial movement of rotating seal ring 30 outboard from flange portion 26.
A annular stationary seal ring (or “primary ring”) 36 is mounted on an annular carrier 38 which is slidably located between annular member 34 and an annular retainer 40 which is secured to housing 18. In the depicted seal, a plurality of radially spaced compression springs 46 act as carrier biasing members between retainer 40 and carrier ring 38, to urge the second stationary seal ring 36 towards the rotating seal ring 30. In other seals known in the art, other carrier biasing mechanisms, such as bellows, are used.
A seal interface 54 is defined by an outboard facing rotating seal face 50 of rotating seal ring 30 and an inboard facing stationary seal face 52 of stationary seal ring 36. Rotating seal face 50 can comprise a grooved area, such that the rotation of shaft 12 will cause seal gas to be pumped between the faces 50 and 52 of seal rings 30 and 36, to generate a load which opposes that applied by the springs 46, creating a fluid cushion to lubricate the sealing faces and provide a seal.
Seal assembly 10 can also include a second stage seal, including a third and fourth seal face member, which are not described here. Further stage seals and seal face members can also be provided.
If seal gas leaks into the seal at high-pressure, the drop in pressure can cause it to liquefy, which can be detrimental to the seal itself. Therefore, in many applications, large and expensive gas conditioning units are required in order to remove contaminants and return any fluids to a gaseous state before the gas is introduced to the seal. In addition, leaking gas must often be vented to the atmosphere, which may lead to undesired or unacceptable environmental consequences.
One approach to preventing leakage in standby conditions, as discussed in U.S. Pat. No. 6,905,123 (which is incorporated by reference herein), provides a stand-still seal in the form of a piston, forced by gas pressure into and out of sealing engagement with a sleeve member. This arrangement requires a source of gas under pressure, such as an accumulator. In addition, the seal chamber of the compressor must provide a passageway to allow pressurized gas into and out of the piston chamber. Such additional requirements are undesirable, because they add complication and expense to the mechanical seal system and compressor. In addition, many seals are installed into pre-existing compressor housings, which are not compatible to be retrofit to provide a passageway for piston gas.
An apparatus for inhibiting static leakage from a mechanical seal, without the need for an additional gas source to the seal, or other modification of the compressor housing would provide decided advantages.